1. Field of the Invention
The present invention relates to a dielectric antenna using a nonradiative dielectric wave guide (an NRD guide) for use for example in an obstruction detector for preventing car accidents, in a radio transmitter, or in another transmitting or receiving device.
2. Description of the Related Art
The inventors of the present invention have filed Japanese Patent Application No. 7-1506 concerning this type of dielectric antenna. In the following description, an XYZ coordinate system is used in which the center of gravity of a dielectric resonator 14 is the point of origin, the extension direction of a dielectric strip 13 is the X direction, and the direction vertical to the principal plane of a conductor 12 is the Z direction.
A conventional dielectric antenna shown in FIGS. 7-9 comprises a first planar conductor 11, and a second planar conductor 12. Disposed between the first planar conductor 11 and the second planar conductor 12 are a dielectric strip 13 and a dielectric resonator 14 spaced from each other along the X axis. One end 13a of the dielectric strip 13 is connected to a waveguide and a transmission circuit (not shown), and the other end 13b is an open end. Provided in the second planar conductor 12 above the dielectric resonator 14 is a single slot 12a substantially parallel to the X axis. In this way, the dielectric strip 13 is placed between the first planar conductor 11 and the second planar conductor 12, thus forming an NRD guide.
As shown in FIG. 10, a dielectric lens 15 is disposed above the slot 12a, and thus a dielectric lens antenna is formed. The electromagnetic waves transmitted from the waveguide and the transmission circuit to the dielectric strip 13 are propagated within the dielectric strip 13 in an LSM (Longitudinal Section Magnetic) mode with their electric field having components within the YZ plane and their magnetic field having components within the XZ plane. The dielectric strip 13 and the dielectric resonator 14 are electromagnetically coupled, so that an electromagnetic wave of an HE111 mode having electric-field components in the X direction occurs within the dielectric resonator 14. The electromagnetic wave generated in the dielectric resonator 14 is radiated through the slot 12a and the dielectric lens 15.
However, when the broadside directional axis provided by the slot 12a is the Z axis, the radiation within the XZ plane ("H plane") along the length of the slot 12a is within a range of approximately .+-.45.degree. with the Z axis as the center. However, the radiation angle within the YZ plane ("E plane") becomes .+-.90.degree. or more, including electromagnetic field components which are not radiated to the dielectric lens 15 ("spill-over loss").